These teachings relate generally to electromagnetic radiation converting structures, and, more particularly, to the preparation and use of high quantum efficiency primary electromagnetic energy converting structures that are stable, and hence capable of providing sustainable secondary emissions, that is, for long durations at desired wavelengths.
Desirable electromagnetic emissions at specific wavelengths can be achieved by either designing the primary emitter to emit at the desired wavelength, such as, for example, different composition electroluminescent devices, or the primary emitter can emit at a primary wavelength that is shorter, often at a higher efficiency, which emission can then be down-converted to the desired longer wavelength by an energy converting element or material. For example, Photoluminescent Materials (e.g. High Persistence Phosphors, such as those disclosed in U.S. Pat. No. 6,117,362 (Blue), or in U.S. Pat. No. 6,267,911 (Green)), Electroluminescent Materials (e.g. UV or blue Light Emitting Diodes (LEDs), or lasers, etc.), Chemiluminescent Devices, etc., can emit at a shorter primary emission wavelength, which emission can be down-converted to desired secondary emissions having longer wavelengths.
Applications of fluorescent dye-based energy converting compositions are cited in U.S. patent application Ser. No. 11/793,376, filed on Dec. 20, 2005, for down-converting primary emissions of high-persistence phosphors to other longer-wavelength visible emissions. Therein the fluorescent dye-based energy conversion structures are rendered as films which embody both the primary emission source (a High Persistence Phosphor), and the energy conversion element in a multilayer film element, such film structures having the flexibility to be applicable to any object.
Use of short-persistence phosphorescent materials are cited in U.S. Pat. No. 7,151,283 for down-converting UV, blue or green LED, primary emissions into secondary emissions at desirable wavelengths, such as, for example, white light. Therein the phosphorescent conversion element is located within the solid state device.
Uses of organic fluorescent materials for down-converting shorter wavelength radiation emitted by UV, blue, or green LEDs to desired longer wavelength radiation are cited in U.S. Pat. No. 6,600,175. Therein also the fluorescent-dye conversion element is located within the solid state device.
Uses of fluorescent materials for down-converting primary emission from UV, or blue, or green LEDs, are also cited in “High Efficiency Phosphor-Converted Light Emitting Diodes for Solid State Lighting” by Steven Allen, Ph.D. Thesis, Univ. of Cincinnati. For the devices proposed in the latter citation the fluorescent organic fluorescent dye-based conversion element is located outside the LED solid state device.
High-Persistence inorganic phosphorescent material based products can be seen in airplanes, multistoried buildings, institutions, etc. Such products do not use any energy conversion elements, and emit at their primary emission wavelengths that are located in the blue (˜490 nm) or green (˜515 nm) areas of the visible spectrum. Remarkably, in spite of the availability of a broad class of high quantum efficiency organic fluorescent materials that can be used as energy conversion elements to create other colors, such products are not found in the marketplace. Apart from the desirability of having photo-luminescent products emitting in other colors (wavelengths) for aesthetical reasons, there is a need for other emission colors for serving informational purposes. An example is a position and hold bar (stripe) at airports, where both the daytime and nighttime (emissive) colors of such a marking need to be in the yellow region of the visible spectrum.
In the world of semiconductor based electroluminescent devices, such as LEDs, there has been a big push to create white light emitting LEDs. This is generally accomplished by utilizing a UV or blue primary emission LED and down-converting their primary emission into white light with an energy conversion element that is an inorganic short-persistence phosphorescent material, such as cerium doped yttrium aluminate. The inorganic phosphorescent energy conversion element is located inside the solid state device. Inorganic phosphorescent materials, such as cerium doped yttrium aluminate, are generally photolytically stable and hence are capable of providing sustainable emissions over a long lifetime. Although other phosphorescent materials are known, which either alone or in combination are capable of down-converting to other colors, their use has not gained wide acceptance in the marketplace. This is because high efficiency materials that can generate the wide range of colors are not available. Similarly, the spectrum of whites from cool white to warm white cannot be created.
Even though the down-converting properties of organic fluorescent materials have been known for some time, the use of such materials as energy conversion elements has been precluded by their inability to provide sustainable secondary emissions over long periods of time, for either outdoor applications using phosphorescent materials as the primary radiation, or for applications using LEDs or lasers as primary radiation. This is because so far the conversion elements have not been photolytically or thermally stable over the long time horizons required for commercial applications.
Multilayer film structures, such as those cited in U.S. patent application Ser. No. 11/793,376, filed on Dec. 20, 2005, can be deployed in a manner wherein the primary emission source, such as the high persistence phosphorescent material, can be located within the film structure, either as a separate layer or within the same layer wherein the organic fluorescence conversion element is located. However, testing reveals that the structures cited do not have long term stability, either for multiple year outdoor usage or for sustainable emissions for lifetimes that are generally desired for solid state devices.
Similarly, fluorescent organic energy conversion elements, such as those cited in U.S. Pat. No. 6,600,175, wherein the conversion element is located within a solid state device, are not capable of sustained emission over the lifetime that is generally required for solid state devices due to degradation of the conversion elements. It should be noted that inside the LED both the light intensity and temperatures are high (temperatures typically exceed 100° C. and can reach 150° C.), thereby accelerating both photolytic and thermal degradation of the organic fluorescent energy conversion elements.
Locating the conversion element remotely from the solid state device, such as an LED, will generally result in its experiencing a lower operating temperature. This may benefit emission sustainability by retarding the rate of degradation of the material. Nevertheless, even with remote location, the organic fluorescent energy conversion element is not capable of sustained emission over lifetimes typically required of solid state devices due to degradation. Remote location of the energy conversion element would also permit greater form factor flexibility by facilitating creation of different form factor lighting devices. Thus, depending upon the application, whether the film based energy conversion elements are located within or remotely from the solid state device, it would be beneficial to have a film based energy conversion structures that allow for sustained emissions.
Fluorescent or phosphorescent conversion elements, such as those cited above, are Lambertian emitters and therefore emit light in all directions. Since the emissions are generally viewed from one direction, it can be appreciated that it would be beneficial to redirect the emitted light towards the viewer. In cases where the film-based energy conversion structures are exposed to the environment, it would also be beneficial to provide clear protective layers for protecting the energy conversion materials from the environment.
Today's white light LEDs utilize inorganic phosphorescent materials that are specific for energy conversion of primary radiation (typically blue or UV) to white light. The use of fluorescent energy conversion elements, such as organic fluorescent dyes, has the potential to increase the conversion efficiency compared to that achieved with phosphorescent materials. Even with the incentive for attaining higher energy conversion efficiency, the use of fluorescent dye-based conversion elements has been precluded for generating sustained emission because of instability of these energy conversion elements, that is, sustained emissions cannot be achieved.
There is, therefore, a need for utilizing higher quantum efficiency energy conversion elements, such as organic fluorescent dyes, that are rendered in structures which are stable and capable of sustained emissions over the lifetimes desired for various applications. It is also desirable, therefore, to provide for energy conversion elements rendered as multilayer structures, wherein the structure not only embodies elements for achieving the high efficiencies in converting the primary emission, but also embodies elements that can substantially increase both photolytic and thermal stability of energy conversion elements, so as to enable sustained emissions over long periods of time. Furthermore, it is also desirable to incorporate within these structures layers that provide the means to direct the Lambertian emissions forward from the conversion elements into the hemisphere from which the emissions will be viewed, as well as protect the conversion elements, both physically (abrasion, etc.) and chemically (solvents, moisture, etc.), from the environment.
Energy conversion elements that provide sustained emission over long periods of time can have substantial utility in a number of different areas, such as in conjunction with primary emissions emanating from short or long persistence phosphors, electroluminescent devices, such as fluorescent tubes, LEDs, lasers, chemo-luminescent devices, LCDs etc. Such elements can be used not only for providing illumination in consumer and industrial applications, but also for displays. Furthermore, these energy conversion elements can also be used for authentication of items such as, credit cards, identification cards, transit passes or any item requiring authentication. There is a need for utilizing these energy conversion elements that are rendered in multilayer structures in order to provide unique charging/discharging and emission characteristics that can be tailored at the site of manufacture and that are difficult to counterfeit.